Coatings may be applied to surfaces of substrates in the form of liquid, paste or powder products, by various methods and equipment, in layers, forming adherent films on the surface of the substrate. Film formation can occur physically or chemically. Physical film formation from liquid coating (wet coating) is known as drying. Drying is typically associated with evaporation of organic solvents or water. Chemical film formation proceeds by chemical reaction between the components of the coating, wherein such reaction can be initiated by energy (heat or radiation) after application of the coating. Physical and chemical film formations are often combined. In such coatings solvent evaporation is followed by film curing (cross-linking of polymer chains).
In the process of drying of coatings, heat is usually supplied to evaporate the coating solvents. The elevated temperatures may also induce chemical drying thereby facilitating a continuous film formation, for example by coalescence of resin particles or by thermally-induced cross-linking of a polymer, comprised in the coating compositions. Heat may be supplied through convection by hot air that is blown on the coated surface. Alternatively or additionally, heat may be supplied through electromagnetic radiation, such as infrared or microwave radiation, wherein the electromagnetic radiation is absorbed by the irradiated substance and/or the coating. Infrared heaters, which can supply large amounts of energy in a limited space and over short periods of time, are, however, rarely used due to their high operating costs and safety concerns, when utilized for evaporation of organic solvents. Even when employing radiation for heating, some forced air displacement is usually needed to carry off the evaporated solvent.
J.P. Patent Application No. 10109062 is directed to a drying method of a coating film, applied from an organic solvent system or a water system, wherein it is the method of irradiating with and carrying out stoving of the near infrared and this near infrared is what is irradiated from a light source which has the spectral distribution which has maximum strength in wavelength of 0.8-1.2 micrometers, and strength decreases to 40% or less of maximum strength on wavelength of not less than 2.0 micrometers.
KR Patent Application No. 20110094730 is directed to a manicure dryer, provided to rapidly dry manicure applied on nails by generating ultrasonic wave and ultraviolet ray, wherein the upper body of the dryer is equipped with an infrared generator, and wherein the infrared generator dries manicure in a short time by evenly emitting heat to the manicure.
The process of drying a nail polish applied to a living tissue cannot afford high temperatures used for drying of coated articles. Thus, the drying should be made effective by alternative methods, such as, for example, high rate air flow or application of photo-sensitive coatings. Several devices have been developed to expedite the nail polish drying process.
U.S. Pat. No. 7,162,811 discloses a method for drying polish applied to the nails of an extremity, the method comprising: applying polish to the nails of an extremity; blowing warm air at a temperature of eighty five degrees Fahrenheit onto the nails for four minutes; then blowing cold air at a temperature of thirty five degrees Fahrenheit onto the nails for two minutes; and then blowing warm air at a temperature of eighty-five degrees Fahrenheit onto the nails for a period of fifteen seconds.
Various photo-curable nail polish formulations have been developed. U.S. Pat. No. 5,435,994 is directed to a photo-reactive coating for application over and for binding with nail polish upon exposure to ultraviolet radiation, comprising: (a) a base resin consisting of nitrocellulose; (b) a photo-reactive monomer selected from the group consisting of methacrylates, dimethacrylates, and mixtures thereof; (c) a photoinitiator consisting of benzyl diketal; and (d) an inhibitor to polymerization.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the figures.
The main ingredients of wet coating are a film-former, capable of forming a film on a surface of a substrate following application of the coating, and a solvent, required to disperse the film-former and to allow its application to the substrate, wherein the solvent is usually evaporated after the coating is applied. Solvents are typically characterized by a high vapor pressure, for allowing evaporation of the solvent at relatively low temperatures. Evaporation may be accelerated by removing the solvent vapor from above the surface of the coated substrate, thereby reducing the solvent's partial pressure in the immediate surrounding atmosphere of the surface. Evaporation may further be accelerated by increasing the temperature of the coating, thereby elevating the solvent's vapor pressure. Often, a combination of both heating and airing the coated surface are applied.
Hot-air type drying devices for the purpose of drying under heating may require high temperature of hot air and a large quantity of air drift at a high flow speed, since heat is transmitted through convection. This means that the surface of a substance to be dried is subject to high temperatures for a long period of time and only the surface portion is over-dried due to rapid drift of air, thereby preventing the underlying layers from being uniformly heated and dried. As a result, a problem may arise in the frequent occurrence of deformation such as a wrinkled coating or a coating in a form of waves, degeneration, dis-coloration, etc. Moreover, when heat is applied, the solvent, the film former and in many cases the coated substrate itself, are all heated to substantially the same temperature, as the coating and the substrate, at least within the layers thereof adjacent to the coated surface, are substantially in thermal equilibrium. In most cases, heating the mass of the coating together with at least a part of the mass of the coated substrate for the sake of heating just the solvent imposes low energetic efficiency on the process.
Thus, there exists an unmet need in the art for a coating drying technique, which would allow selective heating of a solvent in a coating and therefore provide fast and uniform evaporation of the solvent, allowing an effective drying of the coating.